Feedback method and system for electromechanical devices



Sept. 5, 1944. F. G. ALBIN FEEDBACK METHOD AND SYSTEM FORELECTRO-MECHANICAL DEVICES Filed May 26, 1943 MQW ATTo-RNEY.

Patented Sept. 5, 1944 FEEDBACK DIETHOD AND SYSTEM FOR ELECTROMECHANICALDEVICES Frederick George Albin, Los Angeles, Calif., assignor to RadioCorporation of America, a corporation of Delaware Application May 26,1943, Serial No. 488,606

15 Claims.

This invention relates to electrical control circuits and particularlyto a feedback circuit arrangement for a device for translatingelectrical energy into mechanical motion.

In the art of sound recording, a Well-known method of and system formodulating a light beam being impressed upon the sound track area of alm is by the use of a galvanometer having a moving armature which tiltsa mirror for deflecting the beam with respect to the film. The armatureof the galvanometer is actuated by a magnetic field produced by a coilconnected to the output of the speech or audio amplifying equipmentwhich in turn may be connected to a microphone or other translatingdevice such as a disc or film phonograph pickup. The armature of thegalvanometer is usually tensioned, damped, and otherwise influenced toprovide a displacement proportional to the amplitude of the currents atall frequencies, but such influencing introduces a mechanical resonancewhich abnormally increases or decreases the movement in one or morefrequency bands in the audio spectrum. Also, there may frequently be oneor more electrical resonant points somewhere in the system which altersthe optimum proportionality.

A typical galvanometer frequency response curve is illustrated in WolfePatent No. 2,270,367, of January 20, 1942, wherein one form ofcorrection system is disclosed and claimed. In view of these conditions,distortion of the sound being impressed upon the nlm is obtained, thistype of distortion being eliminated only when the overallfrequency-response characteristic curve of the fgalvanometer is flatover the entire audio frefquency range. The present invention reducesthis distortion of the galvanometer to a minimum and thereby permits ofgreater tolerances in the .manufacture of the galvanometer and theelectrical circuit used therewith. The operation of the invention issuch that only those voltages which are directly and solely proportionalto the armature movements are detected and fed back into the speechinput amplifying system.

In addition to the above-mentioned Wolfe patent, other feedback systemsfor eliminating distortion in electro-mechanical devices are known, suchas disclosed in Wilhelm Patent 2,194,175, of March 19, 1940. In order toobtain a voltage truly representative of the motion of the movablemember, Wolfe suggests no inductive coupling between the actuating coiland the pickup coil, while Wilhelm neutralizes the mutual inductance bya transformer or he employs a separate generator for the feedbackvoltages. It has been (Cl. 17g-100.3)

found. however, that because of the peculiar saturation condition ofiron core material encountered in galvanometers and certain otherelectromechanical devices at high current levels. that transformerneutralization or separate generation as shown by Wilhelm is not whollysufficient to produce the desired feedback voltages. Also, that it isdiilicult to practically prevent all mutual inductance between coils onthe same core as shown by Wolfe.

The present invention overcomes these difliculties by the generation ofthe feedback voltages with a second galvanometer identical with themodulating galvanometer but Whose armature is held stationary. In thismanner, the voltage introduced by the mutual coupling between the twocoils on the galvanometer winding may be exactly balanced out bycombining these two voltages in amplitude and phase opposition beforeintroduction into the speech input equipment. The modulating coils ofthe two galvanometers are connected in series aiding while the pickupcoils are connected in series opposing. Potentiometers are provided foreach pickup coil to balance amplitudes While variable resistances areprovided for balancing the phase. By the use of two identicalgalvanometers the exact voltages will be generated in each pickup` coildue to the mutual inductance between the coils on the same core.

The principal object of the present intention, therefore, is to improvethe operation of an electro-mechanical device.

Another object of the invention is to maintain a constant linearrelationship between the amplitude of the input voltage and themechanical displacement of an element actuated thereby at all inputfrequencies.

A further object of the invention is to provide a feedback system for asound recording galvanometer wherein the feedback voltages are generatedin two identical galvanometers.

-A further object of the invention is to re-introduce into an amplifyingsystem, voltages generated by armature movements which are not directlyproportional to the electrical currents originating the movements.

Although the novel features which are believed to be characteristic ofthis invention will be pointed out with particularity in the appendedclaims, the manner of its organization and the mode of its operationwill be better understood by referring to the following descriptionreadin conjunction with the accompanying drawing forming a part hereof, inwhich:

Fig. 1 is a diagrammatic view of a sound record ing system embodying theinvention, and,

Fig. 2 is a graph showing the characteristics o one element of saidsystem.

Referring now to Fig. l, a microphone 5 is connected to an amplier 6which, in turn, is connected to an amplifier 3 feeding the modulatingcoils of two identical galvanometers A and B over conductors I2, i3, andi4. The structures of these galvanometers may be that of any standardtwo coil type such as disclosed and claimed in Dimmick U. S. Patent No.1,936,833, of November 28, 1933. The galvanometer A is the galvanometerfor modulating the light beam in a sound recording system showndiagrammatically by a light source il, a lens and mask unit I6, anoptical slit assembly 20 and a iilm 2l, the light being deected by themirror i9 of the galvanometer A. The modulating coils connected to theoutput of amplifier 8 are connected in series/aiding relationship, thesewindings being mounted on respective armatures or cores l5 and i6, coreI5 being movable for actuating the mirror I9 While core I6 is locked ina stationary position.

A second detecting or pickup winding 22 is shown on core l5 and a secondpickup winding 23 is shown on core I6, these windings being connected ina series opposing relationship. Across the winding 22 is connected apotentiometer 25 and a variable resistor 26, while across winding 23 isconnected a potentiometer 29 and a variable resistor 30, one of theconductors 24 and one of the conductors 28 being interconnected byconductor 21. The potentiometers 25 and 29 are for the purpose ofbalancing the amplitude of the voltages generated in the windings 22 and23, respectively, while the variable resistors 26 and 30 are provided tobalance the phase of the voltages generated in the respective windings.

The output of potentiometers 25 and 29 is fed over conductors 32 and 33to an equalizer 35 over a variable resistor 34 for controlling themagnitude of the feedback amplitude. The output of the equalizer 35 isconnected over conductors 31 to amplifier 8.

In the above arrangement, the voltage introduced in the winding 22 ofthe modulating galgalvanometer is a vector sum of the voltage induced bythe mutual coupling with modulation coil I of galvanometer A plus thevoltage induced by the motion of the armature modulating the Iluxthrough the winding 22. It is only the voltage due to the motion of thearmature or core I which should be fed back out of phase with theoriginal voltages to minimize distortion. rIhis voltage is generallydiilicult to obtain because the peculiar variation in the saturation ofthe core makes the mutual coupling voltage difficult to eliminate. Thislatter voltage can be accurately balanced or neutralized, however, bygenerating another mutual coupling voltage with a galvanometer of thesame construction, but one in which no motion voltage is generated. Thisis accomplished by locking the armature or core I6 of galvanometer Bstationary as shown, the constuction of galvanometer B being identicalto that of galvanometer A. By adjustment of potentiometers 25 and 29 tobalance the amplitudes of the mutual induced voltages, and by adjustmentof resistors 26 and 30 to balance the phase of these voltages, a voltagedue solely to the exact motion of armature l5 is impressed on theequalizer 35, the amplitude of which 'is controllable by variableresistor 34.

The equalizer 35 is for the purpose of restoring the originalfrequency-response charactertic to the galvanometer which is destroyedby the feedback action. It is well-known that in any electromagneticdevice for translating electrical currents into mechanical motion, thevelocity of the motion is constant and independent of frequency for agiven amplitude. That is, the product of frequency and amplitude isconstant and when the frequency of a constant amplitude current goes upthe amplitude of motion or displacement goes down. In constructing agalvanometenthis displacement is made proportional to amplitude at allfrequencies by reducing the efficiency of the device in the lowerfrequency range of the audio spectrum.

When-feedback is introduced into such a system, however, the deviceagain reverts to a constant velocity action and in order to overcome orcorrect for this reversion, an equalizer is provided having acharacteristic of the reverse nature to to that of the characteristic ofthe galvanometer. Such an equalizer is provided at 35, but may also beprovided between the amplifiers 6 and 8 as well.

To illustrate this feature, reference is made to Fig. 2 wherein thehorizontal dot and dash line shows the desired characteristic of theequalizer. When feedback is introduced, the galvanometer functions as aconstant velocity system and the drooping characteristic shown by thesolid line is the result, the rate of amplitude decrease with frequencybeing 6 db. per octave. To restore the fiat frequency-responsecharacteristic, the equalizer 35 is provided with the reversecharacteristic shown by the dotted curve whose variation is also at therate of 6 db. per octave. The above system, therefore, produces feedbackvoltages directly proportional to the amplitude of displacement of themodulating galvanometer armature and an over-al1 nat frequency-responsecharacteristic. Although the invention is illustrated in connection witha sound recording galvanometer, it is to be understood that it isapplicable to any electro-mechanical device such as a loud speaker orother similar device.

I claim as my invention:

1. In a system for translating electrical energy into mechanical energy,a source of electrical energy, a rst translator of said electricalenergy into mechanical energy, said translator having a movable element,means for impressing said electrical energy on said translator forproducing motion of said element, a second translator similar to saidfirst translator, means for simultaneously impressing said electricalenergy on said second translator, the normally movable element of saidsecond translator` being locked in a stationary position, means forgenerating voltages in phase opposition in each of said translators,means for combining said generated voltages, and means for feeding saidcombined voltages into said impressing means.

2. An electrical system in accordance with claim 1, in which saidimpressing means Comprises a modulating coil for each of saidtranslators connected in a series aiding relationship and saidgenerating means comprises a second coil for each of said translatorsconnected in a series opposing relationship, means being provided acrosssaid generating coils for balancing the amplitude and phase of saidgenerated voltages.

3. An electrical system in accordance with claim 1, in which means areprovided in said feeding means for varying said generated volt-2,857,623 age as applied to said impression means at the rate of 6 db.per octave.

4. An electro-mechanical system comprising a source of electricalenergy, a translator having a moving element for translating saidelectrical energy into motion, the motion of said element beingsubstantially proportional to the amplitude of said electrical energy atall frequencies in the audio spectrum, a second translator of a similarconstruction to said iirst translator, the normally moving element ofsaid second translator being adapted to be locked in a stationaryposition, means for impressing said electrical energy simultaneously onboth of said translators, a coil in each of said translators forsimultaneously conducting said electrical energy thereto, a second coilon each of said translators adapted to have voltages induced therein,the second coil in said iirst translator having voltages induced thereinin accordance with the mutual inductance between said coils in saidtranslator and the motion of its moving element and the second coil insaid second translator having voltages induced therein in accordancewith the mutual inductance between the coils in said second translator,means for combining the voltages induced in said second coils on saidtranslators, and means for feeding said combined voltages to said irstmentioned coils in said translators.

5. An electro-mechanical system in accordance with claim 4 in which saidf lrst mentioned coils are connected in a series aiding relationship andsaid second coils on said translators are connected in a series opposingrelationship.

6. An electro-mechanical system in accordance with claim 4 in whichmeans are connected across `said second coils of said translators forbalancing the amplitude and phase of the voltages generated therein inaccordance with the mutual inductance between the coils in therespective translators.

7. An electro-mechanical system in accordance with claim 4, in which anequalizer is provided in said feeding means for varying the frequencyresponse characteristic of saidvoltages in a ratio of 6 db. per octave.

8. The method of maintaining a fiat frequency response characteristic inan electro-mechanical device comprising impressing electrical energy onan operating electro-mechanical device and simultaneously on a similarelectro-mechanical device locked to prevent its operation, generatingvoltages in each of said electro-mechanical devices, the voltagesgenerated in said operating electro-mechanical device being proportionalto the mutual inductance therein and the motion of lits armature, andthe voltages generated in said non-operating device being proportionalto the mutual inductance therein, balancing the generated voltages dueto said mutual inductances and impressing said resultant voltage on saiddevices. y

9. The method in accordance with claim 8, in which the resultantgenerated voltages are varied at the rate of 6 db. per octave beforeimpression on said electro-mechanical devices.

l0. The method of obtaining mechanical motion of the moving element ofan electro-mechanical device proportional to the amplitude of thevoltages impressed thereon at all frequencies within the range oioperation comprising generating electrical voltages, translating saidvoltages into movements of a mechanical element, generating a voltagecorresponding to the said move-- ments and in accordance with the mutualinductance in said device, generating a second voltage correspondingonly to the mutual inductance in said device, balancing said voltagescorrespond ing to said mutual inductance, and combining the voltageresulting from said balancing with said originallygenerated voltages.

11. The method in accordance with claim 10 in which the amplitude ofsaid voltages corresponding to said movements are increased as thefrequencyincreases at the rate of substantiallir 6 db. per octave. n

12. A system of translating electrical energy in'to mechanical motioncomprising a signal source, a irst translator having an actuating coil,a pickup coil and an armature adapted to move proportionally with theamplitude of said signal,

a second translator having an actuating coil, a pickup coil and anarmature locked in a stationary position, means for connecting saidsignal source with said actuating coils, and means for connecting saidpickup coils to said connecting means between said signal source andsaid actuating coils.

13. A system in accordance with claim 12 in which said actuating coilsare connected in a series aiding relationship and said pickup coils areconnected in a series opposing relationship.

14. A system in accordance with claim 12 in which said last mentionedconnecting means includes a potentiometer connected in shunt across eachof said pickup coils for balancing the amplitudes of the voltagesgenerated therein and a variable resistor connected in shunt across eachor `said pickup coils for balancing .the4 phase of the voltagesgenerated therein.

15. A system in accordance with claim' 12 in which said last mentionedmeans includes an equalizer for said generated voltages and means forcontrolling the amplitude thereof.

FREDERICK G. ALBIN.

